MATSEC SEC Physics common mistakes
MATSEC SEC Physics · Understand the traps and how to avoid them.
When converting units before substitution, the candidate multiplies where division is needed (for example converting 250 g to 2.5 kg instead of 0.25 kg, or 20 cm² to 0.2 m² instead of 0.002 m²), then substitutes the wrong value into the formula.
The candidate has memorised 'divide by 1000 for grams' as a rule of thumb but has not built a sense of scale, so the answer is not checked against everyday magnitudes. In area and volume conversions the candidate also forgets that the factor is squared or cubed.
What to do instead
Insist on a two-line conversion habit: write the conversion as a fraction with units (250 g × 1 kg/1000 g) and cancel units explicitly. For areas and volumes, convert the linear dimension first and then square or cube it. Finish every numerical answer with a plausibility check: a mass of a few hundred grams is about 0.1–1 kg, and a room floor is a few tens of m², not thousands.
The candidate writes a final answer with no unit, or with the wrong unit (for example 'the force is 50' or 'the speed is 5 N'), and loses the mark even when the arithmetic is correct.
In the rush of the last minutes the candidate treats the unit as decoration. The candidate also confuses the unit of a derived quantity with the unit of one of the inputs, for example giving work in newtons.
What to do instead
Train a fixed answer template: value, then unit, then a short phrase naming the quantity ('F = 50 N'). Before the exam, build a one-page table of derived units (N, J, W, Pa, C, V, Ω, Hz) written in base units so the candidate can see why each arises. Practise marking your own past answers with a rule that a bare number scores zero.
The candidate treats a distance–time graph as if it were the path of the object and describes the motion as 'going up a hill', or reads the slope of a distance–time graph as acceleration.
The word 'graph' triggers a picture-matching reflex from mathematics lessons, and the candidate has not separated the abstract axes from a physical route. The candidate also mixes up the gradients: gradient of distance–time is speed, gradient of speed–time is acceleration.
What to do instead
Make the candidate label every axis with its quantity and unit before reading anything, then state aloud 'this graph shows how distance changes with time'. Use a paired exercise: same journey drawn as a distance–time graph and as a speed–time graph, with the candidate describing each in words. Drill the two gradient rules and the area rule (area under speed–time = distance) until they are automatic.
When asked to explain why a moving object keeps going, the candidate writes 'because there is a forward force pushing it' rather than identifying the resultant force as zero, or the candidate adds an extra 'motion force' to a free-body diagram.
Everyday language ('the ball is moving so something must be pushing it') conflicts with Newton's first law. The candidate has learned the formula F = ma but not the idea that a resultant force is needed only to change motion, not to maintain it.
What to do instead
Require a free-body diagram for every force question, with only real forces drawn (weight, normal contact, friction, tension, applied push) and an explicit statement of the resultant. Have the candidate work through the constant-velocity case and the accelerating case side by side, writing 'resultant = 0, so constant velocity' and 'resultant ≠ 0, so acceleration'. Ban the phrase 'force of motion' in written answers.
The candidate uses the total weight of a balanced beam or seesaw in the moment equation, or measures a perpendicular distance along the beam rather than perpendicular to the line of action of the force.
The candidate has learned 'moment = force × distance' as a slogan and does not connect the distance to the pivot and the force direction. Symmetry makes the total-weight shortcut look plausible when the beam is uniform.
What to do instead
Insist on marking the pivot with a dot and drawing each force as an arrow with its line of action extended. Then draw and measure the perpendicular distance from the pivot to that line. For a uniform beam, treat the weight as acting at the centre of mass and show it as one force among the others in the sum of clockwise and anticlockwise moments.
The candidate calculates efficiency as output/input × 100 but then adds the 'lost' energy to the output as well, or reports an efficiency above 100% without noticing, or confuses power with energy in the final line.
The candidate is juggling three related quantities (energy, power, efficiency) and has not fixed which is which. Time pressure means the final arithmetic is never checked against the definition.
What to do instead
Write the definition chain explicitly: energy is measured in joules, power is energy per second in watts, efficiency is useful output energy divided by total input energy. Work one example where a motor takes 500 J and delivers 300 J, and have the candidate state the 200 J loss separately and confirm 60%. Make the candidate circle any answer above 100% as an automatic error to recheck.
For a wave, the candidate states that increasing the frequency increases the speed of the wave in the same medium, or mixes up frequency and period when using v = fλ.
The candidate has memorised v = fλ as a triangle and treats all three symbols as freely adjustable. The candidate has not met the idea that wave speed is set by the medium.
What to do instead
Anchor the rule with a concrete case: sound in air travels at about 340 m/s whatever the pitch, so a higher frequency means a shorter wavelength. Practise the relationship both ways, including period T = 1/f, and require the candidate to write the unit of each symbol under the formula before substituting.
The candidate draws a ray diagram for a converging lens or a plane mirror with the normal in the wrong place, angles measured from the mirror surface rather than the normal, or the image drawn on the same side as the object for a plane mirror.
The candidate copies a remembered sketch instead of applying the two rules (angle of incidence equals angle of reflection, measured from the normal; image in a plane mirror is virtual, behind the mirror, same size and distance). Diagrams are often skipped in revision because they feel like drawing rather than physics.
What to do instead
Make the candidate construct at least three ray diagrams per revision session with a ruler and protractor, always drawing the normal as a dashed line first. Mark the angles with arcs on both sides of the normal so the equality is visible. For the plane mirror, require the virtual image to be drawn dashed behind the mirror with equal distances labelled.
In a specific heat capacity calculation, the candidate substitutes the final temperature instead of the temperature change, or uses the mass of the container as well as the water without being asked, or omits the unit J/(kg °C).
The formula Q = mcΔθ is recalled as a string of letters, and Δθ is silently replaced by the hotter reading. The candidate also does not picture the experiment, so the role of each mass is unclear.
What to do instead
Have the candidate write Δθ = θ_final − θ_initial as a separate line with the numbers substituted before touching Q = mcΔθ. In the experiment context, label the thermometer, the water and the container on a sketch and state which mass is being heated. Require the unit of c to be written from the definition: energy per kilogram per degree.
The candidate connects an ammeter in parallel with a component or a voltmeter in series, or adds series and parallel resistors using the wrong rule, and then reports a current that is physically impossible for the supply.
The candidate has memorised 'ammeter in series, voltmeter in parallel' as words without a circuit model, and the resistance rules are recalled as two similar-looking formulas that get swapped under pressure.
What to do instead
Build circuits or use a circuit simulator so the candidate sees the ammeter reading fall to near zero when placed in parallel and the lamp go out. For resistors, derive the series and parallel rules once from the idea that series currents are equal and parallel voltages are equal, then practise a mixed network with a labelled sketch. Always check the total current against V/R for the whole circuit.
The candidate writes 'the pressure increases because the pressure is bigger' or 'the particles push harder because they are hotter', giving a circular answer with no reference to collisions, frequency or force per unit area.
The candidate has memorised the conclusion but not the mechanism, and mark schemes for explanation questions require the chain of reasoning, not the result.
What to do instead
Teach the explanation as a fixed four-step chain: particles move faster, they collide with the walls more often and with greater force, the force on the same area is larger, so the pressure is larger. Have the candidate rehearse this chain aloud for temperature, volume and number-of-particles changes, and mark practice answers against the chain, awarding one mark per step.
In a practical question the candidate reads a measuring cylinder or vernier scale from the wrong part of the meniscus, records a value to more decimal places than the instrument allows, or repeats a reading without averaging.
The candidate has not been taught instrument precision as a concept, so significant figures feel arbitrary. The candidate also treats 'repeat the experiment' as a complete answer without saying what to do with the repeats.
What to do instead
Give the candidate a set of real instruments (ruler, measuring cylinder, thermometer, stopwatch) and ask for the smallest scale division and the correct number of decimal places for each. Establish the habit: read at eye level at the bottom of the meniscus, record to the precision of the scale, and take repeat readings and calculate a mean, discarding obvious anomalies.